This invention is generally directed to new developers and the use of such developers for causing the development of images in an electrophotographic system. More specifically, the present invention is directed to new developers comprised of toners containing charge control agents primarily for the purpose of providing a positive charge on the toner material.
The electrophotographic process is well known as is documented in numerour prior art references including many patents. Generally, the process involves uniformly charging a photoconductive insulating surface which is placed on a conductive backing and subsequently exposing the photoconductive surface to a light image of the original to be reproduced. The photoconductive surface is prepared in such a manner so as to cause it to become conductive under the influence of the light image thus allowing the electrostatic charge formed thereon to be selectively dissipated to produce what is developed by means of a variety of pigmented resin materials specifically made for this purpose such as toner. Such toner materials are electrostatically attracted to the latent image areas on the plate in proportion to the charge concentration contained thereon. Thus, for example, in areas of high charge of concentration there is created areas of high toner density while in corresponding low charge images become proportionately less dense. Thereafter, the developed image is transferred to a final support material such as paper and fixed thereto for permanent record or copy of the original.
Many processes are known for applying the electroscopic particles or toner to the electrostatic latent image to be developed such as for example the development method described in U.S. Pat. No. 3,618,552, cascade development, U.S. Pat. Nos. 2,874,063, 3,251,706, and 3,357,402, magnetic brush development, U.S. Pat. No. 2,221,776, powder cloud development, U.S. Pat. No. 3,166,432, touchdown development. In magnetic brush development for example, a developer material containing toner and magnetic carrier particles is transported by a magnet within the magnetic field of the magnet causing alignment of the magnetic carrier into a brush-like configuration. This so-called magnetic brush is brought into close proximity of the electrostatic latent image bearing surface and the toner particles are drawn from the brush to the electrostatic latent image by electrostatic attraction.
In some instances it may be desirable in electrophotographic systems to produce a reverse copy of the original. Thus, for example, it may be desired to produce a negative copy from a positive original or a positive copy from a negative original. Generally this is referred to in the art as image reversal and in electrostatic printing such image reversal can be affected by applying to the image a developer powder which is repelled by the charged areas of the image and adheres to the discharged areas. More specifically, toners possessing positive charges are found to be very useful and effective in electrophotographic reversal systems and in particular in electrophotographic systems employing organic photoreceptors which in many instances are initially charged negatively rather than positively thus necessitating the need for a positively charged toner.
Reversal developers are described in U.S. Pat. No. 2,986,521, such developers being comprised of electroscopic material coated with finely divided colloidal silica. When this material is used in an electrostatic development system, development of electrostatic images on negatively charged surface is accomplished by applying the electroscopic material having a positive triboelectric relationship with respect to the colloidal silica.
In U.S. Pat. No. 3,893,935 there is described the use of certain quaternary ammonium salts as useful charge control agents for electrostatic toner compositions. According to the disclosure, certain quaternary ammonium salts when incorporated into toner materials were found to provide a particulate toner composition which exhibited relatively high uniform and stable net toner charge when mixed with a suitable carrier vehicle and which toner also exhibited a minimum amount of deleterious toner throw off. U.S. Pat. No. 4,079,014 contains a similar teaching with the exception that a different charge control agent is used, namely a diazo type compound.
Many of the described developers have a tendency to lose their positive charge over a period of time, are difficult to prepare and because of this the quality of the image that is to be developed is adversely affected over a period of time. Further, the use of charge control agents in developers as described in U.S. Pat. No. 3,893,935 are soluble in water causing them to be leached to the toner surface by moisture thereby adversely affecting the machine environment and the copy quality and further such toners containing these materials are humidity sensitive. Additionally these materials are incompatible with the thermoplastic resins and it is very difficult to uniformly disperse or dissolve such materials in the toner. This causes particle-to-particle non-uniformity and wide distribution of electrical charge which in turn reduces the quality of the image developed, and shortens the developer life.
Accordingly, there is a need for developer which can be used in a reverse system and specifically the need for a positively charged toner when used in systems requiring such toners allows the production of high quality images over a long period of use.